Red onion and orange zest make this a beautiful topping for almost any hors d'oeuvre. Make the relish up to a week in advance, and chill, covered; extras are great on sandwiches or with grilled steak.
Heat olive oil in a large skillet over medium heat. Add onion, and cook, stirring occasionally, for 10 minutes or until onion is soft and translucent.
Add remaining ingredients, and cook, stirring occasionally, for 20 to 25 minutes more. (Onion will turn a red, translucent, almost candied color, and most of the liquid will evaporate.) Serve warm, or store in an airtight container in the refrigerator up to a week.